This invention relates to tachometers for monitoring the velocity of the read/write heads in a disk drive, and more specifically to tachometers for monitoring the velocity of a rotary actuator arm of a disk drive.
In general, a closed loop disk drive uses a servo system to accurately position the read/write head at a requested track. In some applications the servo system uses a dedicated disk surface on which servo information is written. In these applications no tachometer is needed, since the dedicated servo surface provides continuous feedback information. Obviously such systems have the disadvantage of having one less disk surface available for data storage, and for systems having one or two disks this represents a substantial storage loss. To alleviate this data storage loss while maintaining the accuracy of the servo system, some applications use a tachometer, coupled to the head actuator to relay velocity information to the servo system. The servo system then uses the head velocity signal as the continuous servo signal, this information being supplemented with fine positioning information stored on a small portion of each sector of each track and normally referred to as embedded servo information.
Generally, a disk drive may use either linear or rotary actuator positioning means. In the linear case, the actuator arm, and therefore the head, is moved linearly along a radius of the rotating disk, while in the rotary case, the actuator arm rotates along an axis parallel to the disk spindle at a point close to the outside rim of the disk. In either case, a suitable tachometer must be used.
Known linear tachometer designs for disk drive applications are not directly applicable to rotary actuator designs. Generally magnetic tachometer designs are very susceptible to stray magnetic fields, and optical tachometers using a glass scale are too expensive.
The available rotary tachometers are not designed for disk drive applications. They range from simple generators that measure the speed of the rotating shaft to more sophisticated optical decoders. Unfortunately they are either not sensitive enough, due in part to the lack of noise rejection and adequate bandwidth, or are too expensive for disk drive applications. Cost is an important factor in the manufacture of reasonably priced disk drives, thus an inexpensive tachometer is needed, provided that the required bandwith and noise rejection can be achieved.